1. Field of the Invention
The present invention relates to a thin-film device comprising an insulating substrate, an insulating film, at least two conductive layers sandwiching the insulating film, said insulating film and said conductive layers being formed on the substrate, one upon another.
2. Description of the Related Art
A thin-film device comprises an insulating substrate, a lower conductive layer formed on the substrate, a thin insulating film covering the lower conductive layer, and an upper conductive layers formed on the insulating film.
The thin-film device is used as multi-layer wiring substrates, thin-film diodes and thin-film transistors.
In most cases, the upper and lower conductive layers (wirings) of the thin-film device are made of metal such as chromium (Cr), tantalum (Ta), molybdenum (Mo), or the like, and the thin insulating film of the device is made of silicon nitride (SiN) or the like. When the thin-film device is manufactured in the form of a thin-film transistor, it has a semiconductor layer of amorphous silicon (a-Si).
A conventional multi-layer wiring board having layers made of a high-melting-point metal such as Cr, Ta, Mo or the like has two problems. First, its manufacturing cost is high since Cr, Ta, Mo or the like is expensive. Second, since the high melting point metal layers have high resistance, the voltage drop in the metal layers is so great as to deteriorate the operating characteristics of semiconductor elements or the like connected to the metal layers.
To solve these problems, it has been proposed that the conductive layer of a multi-layer wiring board be made of aluminum (Al) since aluminum is inexpensive and has a low electrical resistivity. If the conductive layer is made of aluminum, the following problem will arise.
A multi-layer wiring board is manufactured in the following steps. First, a lower conductive layer is formed on an insulating substrate (e.g., a glass substrate). Then, an insulating film is formed on the lower conductive layer. Next, an upper conductive layer is finally formed on the insulating film. The insulating film is formed by a plasma CVD apparatus at a film-forming temperature of hundreds of degrees centigrade. If the lower conductive layer is made of aluminum, it will have hillocks on its surface during the forming of the insulating film. The hillocks protrude into the insulating film, forming defects, e.g., cracks, in the insulating film. Due to the defects, the lower conductive layer will be short-circuited to the upper conductive layer.
Assume the thin-film device described above is used as a thin-film transistor of reverse stagger type. The thin-film transistor is manufactured in the following way. First, a gate electrode (i.e., a lower electrode) is formed on an insulating substrate (e.g., a glass substrate). Next, a gate insulating film is formed on the substrate and the gate electrode. Then, a semiconductor layer is formed on the gate insulating film. Further, a source electrode and a drain electrode (i.e., upper electrodes) are formed on the semiconductor layer. The gate insulating film and the semiconductor layer are formed by a plasma CVD apparatus at a film-forming temperature of hundreds of degrees centigrade. Hence, if the gate electrode is made of aluminum, it will have hillocks on its surface during the forming of the gate insulating film and/or the semiconductor layer. The hillocks protrude into the gate insulating film, forming defects in the gate insulating film. Due to the defects, the gate electrode will be short-circuited to the source and drain electrodes.
In other types of thin-film devices such as a stagger type thin-film transistor, a coplanar type thin-film transistor, a reverse coplanar type thin-film transistor and a thin-film diode, the lower conductive layer, if made of aluminum, will also have hillocks on its surface during the forming of an insulating film, forming defects in the insulating film.
It has been pointed out that an aluminum layer will not have hillocks even if heated to hundreds of degrees centigrade, provided that it contains a small amount of high-melting-point metal (e.g., Ti or Ta). Thus, if the lower conductive layer of a thin-film device is made of aluminum containing a high-melting-point metal, it will have no hillocks on its surface during the forming of an insulating film.
The inventors hereof made a multi-layer wiring board having a lower conductive layer of aluminum containing a high-melting metal. The lower conductive layer and upper conductive layer of this board were short-circuited to each other. To determine why so, the inventors conducted the following experiment.
First, they formed a conductive layer 2 made of Ti-containing aluminum, on a glass substrate 1, as is shown in FIGS. 6 and 7. Next, they heated the glass substrate 1 to about 250.degree. C. and examined the surface condition of the conductive layer 2. No hillocks were formed on the surface of the layer 2, but projections 3 were formed, extending from the sides and edges of the conductive layer 2. The projections 3 were formed, probably because crystal grew locally in the sides and edges of the layer 2 during the layer 2 was heated gradually.
A multi-layer wiring board is manufactured by the method described above. When an insulating film is formed on the lower conductive layer, the lower conductive layer is inevitably heated to the temperature at which the insulating film is formed. Hence, projection will be formed, extending from the sides and edges of the lower conductive layer, during the forming of the insulating film, even if the lower conductive layer is made of aluminum containing a high-melting-point metal. The projections protrude into the insulating film, forming defects, such as cracks, in the insulating film. Due to these defects, the lower conductive layer is short-circuited to the upper conductive layer which is subsequently formed on the insulating film.